An inverter circuit for driving a load such as a motor is an exchanger to switch a direct voltage and an alternating voltage so that the inverter energizes the motor. Specifically, the inverter circuit for driving an inductive motor is composed of, for example, a switching element such as an IGBT (i.e., insulated gate bipolar transistor) and a FWD (i.e., free wheel diode). The IGBT functions as the switching element. The FWD bypasses and refluxes the current flowing in the motor when the IGBT turns off so that the current flowing in the motor does not change even when the IGBT switches between an on-state and an off-state. Specifically, a direct current power source is coupled with the motor. The IGBT applies the voltage to the motor. When the TGBT turns off, the current flowing in the motor flows back through the FWD by the energy accumulated in an inductance L of the motor. Thus, the inverse direct current may be applied to the motor. Since the current in the motor is not blocked immediately when the IGBT switches to the off-state, the alternating voltage is substantially energized from the direct current power source. Since the inverter circuit functions the above operation, it is necessary for the circuit to have the FWD, which is inversely connected in series with the IGBT. Specifically, it is necessary for the FWD to connect inversely in parallel to the IGBT.
A diode used for the FWD is disclosed in U.S. Pat. No. 5,859,446, JP-A-2000-114550, U.S. Pat. No. 6,177,713, JP-A-2002-270857, and JP-A-2000-340806.
FIG. 7 shows a diode 89 disclosed in U.S. Pat. No. 5,859,446. In the diode 89, when the diode 89 intercepts a current, i.e., when the diode 89 is in an interception state, a depletion layer expands in a semiconductor layer 14 having a N− conductive type (i.e., a N− layer 14) so that the current does not flow. When a positive voltage is applied to an anode electrode in the diode 89 with reference to a cathode electrode 17, a hole is introduced from a semiconductor layer having a P+ conductive type 11 (i.e., a P+ layer 11) to the N− layer 14. Thus, the current flows through the diode 89. A semiconductor layer 12 having the P+ conductive type, which is partially formed in a termination region, expands the depletion layer from a junction J1 between the P+ layer 11 and the N− layer 14 to a periphery of the diode 89 so that the semiconductor layer 12 prevents electric field concentration near a boundary of the termination region.
The termination region surrounds an active region and an inductance portion L. An insulation film 13 made of, for example, a SiO2 film is partially formed near the surface of the termination region. A semiconductor layer 15 having a N+ conductive type (i.e., a N+ layer 15) is disposed on a cathode side surface of the N− layer 14. The N+ layer 15 contacts the cathode electrode 17. The N+ layer 15 introduces an electron into the N− layer 14 when a forward voltage is applied to the diode 89.
FIG. 8 is an equivalent circuit diagram showing a semiconductor device 90 suitably used for an inverter circuit of driving the load such as the motor. The device 90 includes an IGBT 90i and a diode 90d, which are inversely connected in parallel to each other.
In the device 90 according to a prior art, the IGBT 90i and the diode 90d are formed in different semiconductor substrates or different semiconductor chips, respectively. However, it is preferred that the IGBT 90i and the diode 90d are formed in the same semiconductor substrate in order to minimize the dimensions of the device.
When the diode 90d is used for the FWD in the inverter circuit, a current waveform of the diode 90d is important in a case where the diode 90d is recovered inversely at a time when the diode 90d switches from the on-state to the off-state.
FIG. 9A shows a circuit for measuring and evaluating a current waveform. Here, the current flows through the diode 90d in the semiconductor device 90. FIG. 9B shows an example of the current waveform.
Semiconductor devices 90a, 90b are provided by the device 90 shown in FIG. 8. An IGBT 90ai in the device 90a is used for the switching device, an IGBT in the device 90b is shunted, and a current Id flowing through the diode 90bd is measured.
As shown in FIG. 9B, when the IGBT 90ai in the device 90a turns off, a circulation current flows in the diode 90bd of the device 90b. When the IGBT 90ai turns on, an instantaneous current flows reversely in the diode 90bd. This instantaneous current has a peak, which is defined as a recovery current Irr. When the diode 90bd recovers reversely, the power source voltage is applied to the diode 90bd. The product of the recovery current Irr and the voltage is defined as a recovery loss. In general, it is required for a rectifier diode to have a small recovery current Irr and a small recovery loss in case of a reverse recovery step. Further, it is required to have slow recovery of the current in case of the reverse recovery step. Thus, it is necessary to improve recovery characteristics of the diode 90bd. 